1. Field of the Invention
The present invention relates to a method for manufacturing abrupt boundary surfaces between ternary nitrides in molecular beam epitaxy.
2. Description of the Prior Art
The material system of the nitrides gallium nitride, indium nitride, aluminum nitride, and the ternary compounds thereof is very well-suited for the realization of optoelectronic components in the spectral region from red to ultraviolet. This is due to the possibility of a direct transition between the conduction band and the valence band (direct semiconductors). The high chemical and thermal resistance of such nitrides enables additional applications in sensor technology and high-temperature electronics as well.
A suitable method for the manufacture of the nitrides is molecular beam epitaxy. According to this method, binary, ternary or quaternary mixed-crystal compositions are deposited in an ultra-high vacuum from material flows of the elements present in the mixed-crystal compositions. For the manufacture of complex components, such as high-efficiency lasers and light-emitting diodes, for example, layer sequences of layers of different mixed-crystal compositions are required. The layers should be of as homogeneous a composition as possible, and the transitions between different compositions of the successive layers should ensue as abruptly as possible. Examples of this include multiple quantum well (MQW) structures in the active region of a laser (e.g., using InGaN) or superlattice structures as buffer layers (e.g., as an alternating sequence of thin layers of AlGaN and GaN). The manufacture of these layers requires an abrupt changeover of the material flows from the material sources for the elements Ga, Al and In in the transition from one layer (already grown) to the next layer (to be grown).
The changeover of the material flows (growth flows) has previously been enabled by one of two methods: interruption of the growth or doubling of the number of material sources. The interruption of the growth between the different layers allows the changeover of the material flows of the material sources, for the elements of group III of the periodic table, by changing the temperatures of the effusion cells of the epitaxy apparatus. A disadvantage of this method is that relatively long periods of time elapse before a source delivers a new stable material flow, which is required for the growth of the following layer. Moreover, the surface of the grown semiconductor layer changes during the interruption.
The other known possibility is to double the number of the material sources. One source then supplies the material flow needed at that moment while the other source is set to the material flow required for the following layer. However, in molecular beam epitaxial apparatuses, the maximum possible number of sources is very limited for geometrical reasons. As such, a doubling of effusion cells severely reduces the possibility of using these apparatuses.